In recent years, in order to realize a light-weight automobile, an application of high-strength steel to a body has advanced, and application of a steel sheet with a tensile strength of over 980 MPa has expanded. In contrast, problems such as a decrease in life duration of a mold used when the component is processed and a greater variation in shapes due to springback occur as the strength increases. Thus, a method has been developed in which deformation resistance is reduced while springback is reduced by heating a low-strength steel sheet at a temperature (approximately 900° C. or more) that is equal to or greater than a transformation point Ac1 before press-forming to austenitize the steel sheet and then forming it in a high-temperature range, while the strength of a formed article is secured by performing quenching while forming, a so-called hot press-forming method (hereinafter, also referred to as a “hot stamp”). Such a hot stamp has been distributed as a method of manufacturing a component (press-formed article) required to have a high strength in a class of 1470 MPa or greater in terms of a tensile strength, in particular.
In contrast, it is essential to provide a sacrificial protection effect to components that are applied to a side member, a side sill, a cross member, a pillar lower part, and the like, which are required to have high corrosion resistance, in the automobile structural members, and cold formed components of galvanized steel sheets or hot-dip galvannealed steel sheet has been applied heretofore. Currently, there is a requirement of a press-formed article with a high strength and high corrosion resistance which can be applied to a side member, a side sill, a cross member, a pillar lower part, or the like and is obtained by forming a galvanized steel sheet or a hot-dip galvannealed steel sheet into a component in a hot stamp process.
FIG. 1 is an explanatory outline diagram illustrating a mold configuration for performing the hot stamp as described above, and in the diagram, 1 represents a punch, 2 represents a die, 3 represents a blank holder, 4 represents a steel sheet (blank), BHF represents blank holding force, rp represents a radius of a punch shoulder, rd represents a radius of a die shoulder, and CL represents a clearance between the punch and the die, respectively. Among these components, paths 1a and 2a through which a cooling medium (such as water) can be made to flow are formed inside the punch 1 and the die 2, respectively, and these members are configured to be cooled by making the cooling medium to flow through the paths.
When the hot stamp (such as hot deep drawing processing) is performed by using such a mold, the forming is started in a state in which the steel sheet (blank) 4 is heated at a two-phase range temperature (from a transformation point Ac1 to a transformation point Ac3) or at a single-phase range temperature of equal to or greater than a transformation point Ac3 and is softened. That is, the steel sheet 4 is press-fitted into a hole (between 2 and 2 in FIG. 1) of the die 2 by the punch 1 in a state in which the steel sheet 4 in the high-temperature state is pinched between the die 2 and the blank holder 3 and is formed into a shape corresponding to an outer shape of the punch 1 while reducing an outer diameter of the steel sheet 4. In addition, heat release from the steel plate 4 to the mold (the punch 1 and the die 2) is performed by cooling the punch 1 and the die 2 in parallel with the forming, and the material is quenched by further cooling and keeping it at a bottom dead center of the forming (a timing at which a tip end of the punch is located at the deepest position: the state illustrated in FIG. 1). By performing such a press-forming method, it is possible to obtain a formed article in a class of 1470 MPa or greater with high dimensional precision. It is also possible to reduce a forming load as compared with a case of cold-forming a component in the same strength class, and therefore, only a pressing machine with a small volume is required.
However, if a galvanized steel sheet or a hot-dip galvannealed steel sheet is subjected to the hot stamp, cracking occurs during press-forming due to liquid metal embrittlement (hereinafter, also referred to as “LME”) in which zinc liquefied (melted) at a high temperature enters a grain boundary of the steel sheet and the steel sheet embrittles, and problems such as a decrease in an impact resistant property and a fatigue strength of the component (press-formed article) occur.
In order to suppress cracking (hereinafter, also referred to as “LME cracking”) due to such LME, PTL 1, for example, proposes a method of suppressing the occurrence of cracking during press-forming processing by setting a high-temperature holding time before the press-forming to be relatively long (300 seconds or longer, for example) to accelerate alloying of a plated layer and to increase Fe concentration in the plated layer. However, according to this method, it is necessary to perform heating and holding for a long period of time in the hot stamp process, and there is a disadvantage that productivity is degraded.